The present invention relates to a superconductive tunnel junction device which is capable of both current and voltage amplification of low-level signals.
The search for a superconductive 3-terminal device with amplifying capability and good isolation of the input from the output has been in progress for more than 20 years. The advantages of superconductive electronics over semiconductor electronics are in two broad areas: digital applications based on the high switching speed and low power dissipation of Josephson junctions, and analog applications based on extremely high sensitivity and response to electromagnetic phenomena over a very wide frequency spectrum. This spectrum ranges from dc and low frequency magnetic fields (where SQUID devices are used), to microwaves and mm waves, infrared, optical, and UV radiation, through to the detection and spectroscopy of X-rays and xcex3-rays. Also lowering the temperature of electronic circuitry tends to reduce its inherent noise. However, without a complementing 3-terminal transistor-like device, these advantages are limited to very specialised applications. In the immediate future, particularly in the application of superconducting photon counting spectrometers to astrophysics, many detector pixels and analogue electronics channels will be required.
In order to exploit these advantages a 3-terminal transistor-like device should fulfil a stringent list of properties including:
current gain;
voltage gain;
isolation between input and output;
high speed;
potential for very large scale integration (VLSI);
low power,
impedances compatible with other devices and with transmission lines;
manufacturability.
For ease of digital design 3-terminal devices should also be inverting and non-latching.
At least three different devices based on non-equilibrium superconductivity and quasiparticle dynamics have been proposed and studied: the Gray effect transistor, the Quiteron, and the quasiparticle multiplier. Each of these will be described briefly below but all of these devices utilise quasiparticle tunnelling.
The Gray effect transistor is described in U.S. Pat. No. 4,157,555 and relies on the fact that there are two tunnelling processes (and their inverses) for quasiparticles in superconductor-insulator-superconductor (SIS) junctions, and under certain conditions multiple tunnelling can occur. The Gray effect transistor is formed by disposing three thin films of superconductive material in a planar parallel arrangement and insulating the films from each other by layers of insulating oxides to form two tunnel junctions. One junction is biased above twice the superconductive energy gap and the other is biassed at less than twice the superconductive energy gap. Injection of quasiparticles into the centre film by one junction provides a current gain in the second junction.
This multiple tunnelling effect when combined with quasiparticle trapping has been used to amplify low-level signals from the absorption of single X-ray quanta. More recently the same scheme with multiple tunnelling amplification factors of up to about 200 has been used to count and measure the energy of individual optical quanta. Although this amplification scheme has proven to be extremely useful, it produces charge amplification, not current amplification, that is to say the length of the current pulses is extended, not their amplitude.
The Quiteron is described in EP-A1-0081007 and produces gain by suppression of the energy gap xcex94 of the central electrode of a double SIS tunnel junction structure. In this device the superconductive energy gap of the central superconductive layer is greatly altered by over-injection of energetic quasiparticles so that the energy gap changes greatly with respect to its thermal equilibrium value, and in most cases is made to vanish. In one example of this device a three electrode device is constructed with tunnel barriers between the electrodes. A first of the tunnel junctions is used to heavily inject energetic quasiparticles into the central superconductive electrode to change its superconductive energy gap drastically. In turn, this greatly modifies the current-voltage characteristics of the second tunnel junction. This device appeared initially to be very promising, but never fulfilled all of the requirements for either analog or switching devices, particularly that of isolation of the input from the output. This problem can be visualised using the two-fluid (Landauer) model of transistor-like operation in which a 3-terminal device is viewed in analogy with a fluid-actuated xe2x80x9cpistonxe2x80x9d in which a xe2x80x9ccontrolxe2x80x9d fluid controls the flow of a second xe2x80x9cmovingxe2x80x9d fluid. For proper operation it is important that the fluids be separate with little mixing between them. For bipolar transistors a small base current controls a much larger collector current and changes in the output have little effect on the input. Similar features apply to most other semiconductor 3-terminal devices. However, for the Quiteron the central superconductive electrode is shared between input and output with only a small degree of unidirectionality. Such a device is not very useful. Moreover, the gap suppression requires a large deviation from equilibrium.
The quasiparticle multiplier [N E Booth xe2x80x9cQuasiparticle trapping and the quasiparticle multiplierxe2x80x9d, Appl. Phys. Lett. 50, 293 (1987)] makes use of the quasiparticle trapping scheme. Like the Quiteron it consists of (a minimum of) three superconductive films separated by tunnelling barriers. The central film is a bilayer of two superconductors, a primary film S1 of energy gap xcex941 and a trapping film S2 of energy gap xcex942. If xcex942 is less than one third of xcex941, additional quasiparticles can be produced by phonons emitted in the trapping process where a quasiparticle diffusing from S1 into S2 relaxes to the lower energy gap. In contrast to the Quiteron, there is a high degree of directionality. Although, as originally proposed, several stages can be cascaded like a photomultiplier, the gain per stage for Nbxe2x80x94Al films is not more than 3, and this value is obtained only if all relaxation phonons are absorbed in the trap, which is highly unlikely.
It should also be said that all three of the devices above require the application of a small magnetic field to suppress Josephson effects.
Other devices which may have specific applications, such as devices based on vortex flow, have also been proposed and studied. Very recently a new device based on controlling a supercurrent by the current through a normal metal film has been proposed A F Morpurgo, T M Klapwijk and B J van Wees, xe2x80x9cHot Electron Tunable Supercurrentxe2x80x9d, Appl. Phys. Lett. 72, 966 (1998)
Thus the need for a satisfactory superconductive transistor-like device has still not been met.
The present invention provides a device in which quasiparticles travelling from a superconductive region with energy gap xcex94 to a normal region lose their potential energy in electron-electron interactions to increase the number of excited electrons in the normal region above the equilibrium thermally excited number. This increase is the basis for current amplification. The same effect can occur with holes. In the following, the term charge carriers will be used to refer to electrons or holes.
In more detail, according to the present invention there is provided a superconductive tunnel junction device comprising a first superconductive region in contact with a first normal region, wherein the potential energy of quasiparticles from the first superconductive region relaxing into the first normal region is converted into an increased number of charge carriers excited above the Fermi level of the first normal region.
The device may further comprise a second superconductive region separated from the first normal region by an insulating tunnel barrier to form a tunnel junction, across which the charge carriers tunnel into the second superconductive region to form quasiparticles therein. This second superconductive region can form an electrode of the device.
The second superconductive region and the first normal region may be biassed relative to one another so that the energy gap of the second superconductive region is near the Fermi level of the first normal region.
Thus the first normal (conductivity) region acts as a trap for quasiparticles from the first superconductive region and when the quasiparticles enter the trap they relax and their potential energy is converted into kinetic energy of the charge carriers in the normal trap. This normal trap is arranged to act as the normal electrode of a tunnel junction which is an NIS (normal, insulator, superconductor) junction. Because of the transfer of energy from the relaxing quasiparticles, the number of charge carriers in the normal trap is increased above the equilibrium number created by thermal excitation and this is the basis of the current amplification. It should be noted that although normal trap regions have been used previously to dissipate the energy of quasiparticles from a superconductive electrode, in the present invention the energy is not dissipated but is used to excite charge carriers in the normal trap which thereby form an amplified current.
An important feature of the superconductive device is the existence of an energy gap in the first superconductive region so that the potential energy xcex94 of quasiparticles in the first superconductive region is converted into the excitation of charge carriers which, in turn, form for the second superconductive region a tunnelling current which flows only when there are excited charge carriers in the normal metal trap.
The quasiparticles in the first superconductive region can be created directly therein by incident radiation or particles (e.g. nuclear particles) when the device is being used as a detector, or can be injected via a tunnel junction from a xe2x80x9cbasexe2x80x9d region. (The terms xe2x80x9cbasexe2x80x9d, as well as xe2x80x9cemitterxe2x80x9d and xe2x80x9ccollectorxe2x80x9d will be used in analogy with the bipolar transmitter though, obviously, the physical principals of operation are different). The base region can comprise a second normal region separated from the first superconductive region by an insulating tunnel barrier. The base region is electrically biassed so that charge carriers excited above the Fermi level tunnel through the tunnel barrier to form quasiparticles in the first superconductive region. Thus the device has an N-I-SN-I-S structure. The amplifying effect can be understood by comparing the number of excited charge carriers in the first normal region (the trap) compared to the second normal region (the base). In the base the number of excited charge carriers is simply the equilibrium thermally excited number. However in the trap that number is increased by the energy from the relaxing quasiparticles from the first superconductive region. That increased number represents an amplification.
In the N-I-SN-I-S device described above with a normal base region, the first normal region can form the emitter electrode and the second superconductive region the collector electrode, thus producing a device analogous to a bipolar transistor.
In alternative embodiments the base region can comprise a superconductive region in which case the base-emitter junction acts as a Josephson junction, or the Josephson current therein can be suppressed by the application of a small magnetic field. Another alternative is for the base region to be a semiconducting flim, e.g. for use as a radiation detector.
The N-I-SN-I-S device described above can be modified by the normal base region carrying a further superconductive region in intimate contact with it so that the base normal film acts as a trap for quasiparticles generated in the further superconductive region. Such a device can be used, again, as a radiation detector, particularly for X-rays or a phonon detector.
The collector region (second superconductor) is electrically biassed relative to the emitter region (first normal region) so that the top of the energy gap is just above the Fermi level of the emitter to allow tunnelling of the charge carriers. Similarly, where a base region is provided, the base and emitter are relatively biassed similarly to place the top of the energy gap of the first superconductive region just above the Fermi level to allow tunnelling of the charge carriers.
Although the second superconductive region can itself form the collector electrode, it is also possible to provide a further normal region in contact with it for acting as a trap for quasiparticles from that region. That normal region then forms the electrical and thermal contact of the collector electrode.
In the above devices the base-emitter junction or the collector-emitter junction can be divided into two or more parallel junctions to provide adding circuits, fan-out circuits or double collector circuits.
As for the materials used in the device, the superconductor can be any superconductive material e.g. niobium, niobium nitride or aluminium and the normal regions can be regions of metal-like conductivity, e.g. metals such as molybdenum, tungsten, magnesium, calcium, copper, palladium, silver or gold. The superconductive materials, particularly where aluminium is used, may be impure or disordered to reduce the superconducting coherence length. Of course, high-temperature superconductors or organic superconductors can be used.
The device can be used as an analog electrical signal amplifier or as a digital switching device in analogous manner to a semiconducting transistor. It can also be used as a detector for a wide variety of particles/radiation, such as nuclear particles, photons, X-rays, phonons, bimolecular ions or dust particles. Thus it can be used in conjunction with various instruments e.g. mass spectrometer, scanning electron microscope.
The device can also be used as a microrefrigeration device because the removal of xe2x80x9chotxe2x80x9d charge carriers from the base region results in electronic cooling of that region. The cooling effect can be increased by the use of two or more devices to cool the same base region.
It should be understood that the device can be biassed so that the charge carriers tunnelling from the normal regions into the superconductors are either electrons or holes.
It will be appreciated, therefore, that the device of the present invention can perform a wide variety of functions and also possesses all of the properties required of a superconducting three-terminal device. It is possible to realise output characteristics equivalent to both the pnp and npn bipolar transistors from the same device, simply by reversing the bias polarities. Also two complementary devices can easily be realised which do not exist in the semiconductor field, namely devices with negative current gain. In contrast to the Quiteron the influence on the input of changes in the output are minimal, thus it has good input-output isolation. The device can also easily be merged with Josephson junction circuits because, unlike the three non-equilibrium devices mentioned in the prior art discussion above, no magnetic field is needed to suppressed Josephson effects. The device can be arranged to accept optically coupled inputs with high sensitivity.
A particular advantage is that it is highly resistant to radiation damage. This makes the device useful in hostile environments such as nuclear reactors or accelerators or space (e.g. aboard satellites).